Fuel cell start-up method, fuel cell system and vehicle equipped with same

ABSTRACT

The invention relates to a fuel cell start-up method, said fuel cell comprising numerous cells which are supplied by a reformer ( 10 ). According to the invention, when the reformer is cold, reformates are supplied to a first sub-assembly ( 12 ) of cells of the fuel cell and, when the reformer is hot, reformates are supplied to the first and second sub-assemblies of cells of the fuel cell. The cells belonging to the first sub-assembly are optimised in order to operate with a cold reformer and the cells belonging to the second sub-assembly ( 13 ) are optimised to operate with a hot reformer.

The present invention relates to the area of fuel-cell stacks and topropulsion systems comprising a fuel-cell stack.

Traditionally, a fuel-cell stack system with gas reforming is capable ofgenerating electric power for supplying an electric motor, for examplefor propelling a vehicle.

The system comprises a reformer with which hydrogen can be generatedfrom a fuel, such as gasoline or methanol carried on board the vehicle,a stack permitting the generation of electric power from the hydrogensupplied by the reformer and from the oxygen of the air, and auxiliaryequipment, especially an air compressor and a cooling circuit.

The fuel-cell stack can be composed of a plurality of cells connected inseries to reach the operating voltage of the power train of the vehicle.These cells are dimensioned as a function of the nominal power that mustbe achieved in the power train.

A cell of a fuel-cell stack comprises a bipolar plate and anelectrode/membrane assembly. The cell consumes hydrogen to form protonsand electrons at the anode. The protons are transferred to anelectrolyte through the membrane, and the electrons are transferred bythe bipolar plates to the electric circuit of the stack output. At thecathode, oxygen of the air combines with the protons and the electronsto form water.

The bipolar plate must ensure uniform distribution of the reagents overthe electrodes, conduct the electrons, evacuate the water producedduring the electrochemical reaction to the outside, evacuate the heatproduced during exothermic reactions and assure gastightness. It can bemade of graphite, conductive polymer, metal, etc. The bipolar plate maybe of the “channel” type or of the “porous” type. The cathode and anodecan be fixed on opposite sides of the membrane constituting electrolyte.The proton exchange membrane ensures flow of protons from the anode tothe cathode and ensures gastightness. It can be made f sulfonatedperfluorinated material such as NAFION, ACIPLEX, etc.

The electrodes, the anode and cathode, ensure supply of the reagents tothe reaction sites, perform the electrochemical reactions and evacuatethe reaction products to the bipolar plates. The electrodes can be madeof a carbon fabric that additionally contains a catalyst such asplatinum for the reactions, as well as hydrophobic agents such as PTFEto evacuate the water. The electrodes and the membrane are intimatelyconnected to ensure transfer of a reagent from one medium to the other,for example by hot pressing of the electrodes onto the membrane, or elseby direct deposition of the electrodes on the membrane.

The chemical energy contained in the gases and not transformed toelectricity reappears in the form of heat. At present, the knownmaterials for the membrane must be maintained at a temperature below 80°C. It is therefore necessary to provide a cooling circuit. A coolingcell can be interposed every one to three cells. The cooling cell has ashape similar to that of a bipolar plate but is used for circulation ofa cooling fluid such as water or water containing an antifreeze product.

The most severe operating conditions for a stack in an automobileapplication occur during the starting phase, where the stack mustproduce electricity as rapidly as possible. Consequently the materialsare subjected to severe thermal stresses, since the temperature risesfrom ambient to operating temperature in a few minutes.

In addition, in order to raise the temperature of the fuel-cell stackrapidly, the cells must operate at low voltages to generate a largequantity of heat required for their temperature rise, and thisnecessitates the use of robust materials.

Furthermore, in the case of hydrogen supply by a reformer, the anodiccatalysts based on platinum alloy used in the stack are very sensitiveto carbon monoxide, which is a byproduct of the reforming reactions.

During starting phases, the reformer must be brought up to temperatureand, during this phase, it produces a greater quantity of carbonmonoxide than during the subsequent operating phases.

For these reasons, the fuel-cell stacks are generally overdimensioned,for example with an extra quantity of catalysts in the electrodes, thussubstantially increasing the cost thereof.

Moreover, in order to improve the cooling of the fuel-cell stack, it isdesired to raise the operating temperature. The materials envisioned fora high-temperature fuel-cell stack have a limited operating range. Forexample, polybenzimadazole (PBI) has good ionic conductivity only attemperatures of 120° C. and above, thus leading to prolongation of thestarting duration of the stack. In fact, it is necessary to raise theentire fuel-cell stack to this temperature before it can begin toproduce electricity.

The very rapid dynamics imposed in an automobile application alsorequire that the fuel-cell stacks be overdimensioned, so that they canrespond rapidly to abrupt power variations.

International Patent WO 00/30200 (Ballard Power Systems) describes afuel-cell stack system provided with a reformer. During a startingphase, the fuel being used to supply the reformer is directed toward atleast one portion of the cells of the fuel-cell stack. By directoxidation of the fuel, the cells of this portion deliver an output powerat least until the reformer is operational. Thus the risk exists thatthe cells supplied directly with reformer fuel will be rapidly degradedeven if great expense is incurred for construction thereof.

The invention proposes to solve this problem.

The invention proposes, for starting the fuel-cell stack, a method thatis fast, economical, and does little harm to the fuel-cell stack.

According to one aspect of the invention, the starting method isintended for a fuel-cell stack comprising a plurality of cells suppliedby a reformer. When the reformer is cold, a first subassembly of cellsof the fuel-cell stack is supplied with reformate. Thereafter, when thereformer is hot, the first and second subassemblies of cells of thefuel-cell stack are supplied, the cells of the first subassembly beingoptimized for operation with a cold reformer and the cells of the secondsubassembly being optimized for operation with a hot reformer.

There is understood by

cold reformer, a reformer that has not reached its optimal operatingcondition, and by

hot reformer, a reformer that has reached its optimal operatingcondition.

In this way it is possible to construct the cells of the secondsubassembly in a manner optimized for high efficiency and low cost,these cells being able to demand hydrogen-rich reformates. Furthermore,the ability of the cells of the first subassembly to operate withhydrogen-lean reformates that may contain a notable proportion of carbonmonoxide or other compounds containing oxygen and/or carbon in additionto hydrogen is optimized.

Such a method makes it possible to render a fuel-cell stack operationalvery quickly, thus removing a very important obstacle to thecommercialization of vehicles equipped with fuel-cell stacks—an obstaclerepresented by the duration that the driver must wait after pressing astarting button until the moment when the vehicle is truly operational,or in other words has sufficient electric power that it can run underappropriate conditions.

In one embodiment of the invention, the cells of the second subassemblyare supplied when the said cells are at an appropriate operatingtemperature. If the operating temperature has not been reached, thecells of the second subassembly are not supplied. The cells of thesecond subassembly can therefore be made with materials having highperformance, especially as regards efficiency, but also meeting highrequirements in the matter of operating-temperature range.

Advantageously, a cooling circuit common to the first and secondsubassemblies of cells of the fuel-cell stack is activated when thetemperature of the first subassembly of cells reaches a temperaturethreshold.

In a first phase, during a cold start, the cooling circuit isdeactivated to ensure that the cells of the first subassembly rapidlyrise in temperature.

Then, in a second phase, the cooling circuit is activated to ensure thatthe cooling fluid circulates between the first and second subassembliesof cells, in such a way that the heat released by the cells of the firstsubassembly is used to bring the cells of the second subassembly up totemperature.

Finally, in a third phase, when the temperature of the cells of thesecond subassembly has reached a temperature threshold, the coolingfluid of the cooling circuit circulates in the cells of the first andsecond subassemblies as well as in a radiator, thus permitting the heatproduced to be evacuated to the outside.

The present invention also proposes a fuel-cell stack system comprisinga fuel-cell stack provided with a plurality of cells and a reformercapable of supplying hydrogen from a hydrocarbon fuel.

The system comprises a first subassembly of cells optimized to operatewith a cold reformer and a second subassembly of cells optimized tooperate with a hot reformer, and means for supplying the secondsubassembly of cells as a function of the reformer temperature.

The invention likewise proposes a fuel-cell stack system comprising afuel-cell stack provided with a plurality of cells and a reformercapable of supplying hydrogen from a hydrocarbon fuel.

The system comprises a first subassembly of cells optimized to operatewhen cold and a second subassembly of cells optimized to operate whenhot, and means for supplying the second subassembly of cells as afunction of their temperature. Thus materials with high operatingtemperature can be used for the second subassembly of cells, whichpermits effective cooling.

There is understood by

operating when cold, operation of the stack from ambient temperature upto the minimum temperature of the second subassembly and with an anodicgas than can be discharged from a cold reformer, and by

operating when hot, operation of the stack in its ideal temperaturerange with an anodic gas discharged from a hot reformer.

Advantageously, the system comprises a cooling circuit common to thefirst and second subassemblies of cells, in such a way that the heatreleased by the first subassembly of cells heats the second subassemblyof cells when the latter is shut down.

In one embodiment of the invention, the system comprises apilot-controlled valve mounted on a reformate-supply conduit of thesecond subassembly of cells, a pilot-controlled valve mounted on anair-supply conduit of the second subassembly of cells, and an electronicswitch mounted on an output conductor of the second subassembly ofcells.

In this way it is possible to interrupt the supply of reformate and ofair to the second subassembly of cells when these operating conditionsare not present in combination. It is also possible to isolate thesecond subassembly of cells electrically.

In one embodiment of the invention, the system comprises a central unitprovided with means to run a software routine, with a memory and with atleast one software routine stored in the memory. The software routinecomprises a module to activate a cooling circuit when the temperature ofthe first subassembly of cells reaches a temperature threshold.

It is understood that the subdivision of the fuel-cell stack into twosubassemblies comprising different materials makes it possible to lowerthe cost of the stack, because the starting stresses are applied to onlyone of the subassemblies. Only that subassembly must contain thematerials that are most resistant to cold operation of the saidsubassembly and of the reformer.

Another advantage of the invention lies in improving the energyefficiency of the fuel-cell stack, because the cells of the secondsubassembly generate electric power only when they have reached theiroptimal temperature range and therefore do so with high efficiency.

A vehicle equipped with an electric propulsion motor and a fuel-cellstack capable of supplying the motor has a high degree of autonomy withgreatly reduced pollution and fast starting.

The present invention will be better understood by studying the detaileddescription of several non-limitative practical examples illustrated bythe attached drawings, wherein:

FIG. 1 is a schematic view of one cell of a fuel-cell stack; and

FIG. 2 is a schematic view of the system according to one aspect of theinvention.

As can be seen in FIG. 1, a cell of a fuel-cell stack comprises amembrane 1 performing the function of electrolyte, an anode 2 disposedon one side of membrane 1 and a cathode 3 disposed on the opposite side,a plate 4 on the side of anode 2 opposite membrane 1 and a plate 5disposed on the side of cathode 3 opposite plate 1. Plate 4 is providedwith channels 6 forming open grooves on the side of anode 2. The same istrue for plate 5, which is provided with open channels 6 on the side ofcathode 3 and with open channels 7 on the opposite side, which can bebrought into contact with the anode of another cell.

In FIG. 2 there is illustrated a system according to one aspect of theinvention. The system comprises a fuel-cell stack 8, an air compressor 9and a reformer 10 supplied with fuel by a reservoir 11. Fuel-cell stack8 comprises two subassemblies 12 and 13. Each subassembly 12, 13comprises one or more cells such as described hereinabove. A conduit 14is mounted between the outlet of reformer 10 and the inlet ofsubassembly 12 in order to supply the said subassembly 12 withreformate. By “reformate” there is understood here the chemicalcompounds produced by the reformer, including hydrogen, the proportionof which is desired to be as high as possible, carbon monoxide, theproportion of which is desired to be as low as possible but whichgenerally increases when the reformer has not reached its idealoperating temperature or else during transient phases, and possiblyother compounds containing hydrogen, oxygen and/or carbon.

A conduit 15 is mounted between the outlet of air compressor 9 andsubassembly 12. A conduit 16 is branched off from conduit 14 to openinto subassembly 13. A pilot-controlled valve 17 is mounted on conduit16. Similarly, a conduit 18 equipped with a pilot-controlled valve 19 isbranched off from air conduit 15 to open into subassembly 13. Theelectrical outputs of subassemblies 12 and 13 are connected in parallel,but nevertheless an electronic switch 20 is mounted on one of theelectrical outputs of subassembly 13 to ensure that it can be isolatedelectrically.

There is provided a cooling circuit 21, which comprises a conduit thatpasses through subassemblies 12 and 13 to ensure cooling thereof and apilot-controlled valve 26, the cooling fluid passing first throughsubassembly 12 and then through subassembly 13. The system alsocomprises a central unit 22 capable of actuating the other elements, inparticular compressor 9, and pilot-controlled valves 17, 19 and 26.Switch 20 is connected to a temperature sensor 23 of reformer 10, atemperature sensor 24 of subassembly 12 and a temperature sensor 25 ofsubassembly 13.

Subassembly 12 is optimized to support, on the one hand, operation atlow temperature and, on the other hand, a hydrogen-lean supply, or inother words a supply containing reformates rich in carbon monoxide fromreformer 10, itself at low temperature.

The cells of subassembly 12 are therefore provided with a quantity ofcatalysts, such as platinum or platinum alloy, that is higher than thatof the cells of subassembly 13, with catalysts adapted to carbonmonoxide poisoning, with a membrane whose low-temperature conductivityis sufficient to deliver adequate electric power immediately uponstarting, and finally with a sufficiently thick membrane.

Subassembly 13 will operate once it has reached its operatingtemperature and once the reformer is delivering reformate with asufficiently low carbon monoxide ratio.

The cells of subassembly 13 can therefore be provided with a smallerquantity of catalysts, with catalysts adapted to the carbon monoxideratio of reformates delivered by the reformer during continuousoperation, with a membrane having excellent ionic conductivity at itscontinuous operating temperature and finally with a membrane of smallthickness.

Upon startup of a vehicle equipped with such a system, a procedure forbringing the reformer up to temperature is activated, for example bymeans of a fuel burner integrated into the reformer. Normally, thereformer begins to produce hydrogen only once it has reached itsoperating temperature, which typically is 800° C.

Nevertheless, the production of hydrogen may begin at much lowertemperature, on the order of 600 to 700° C., albeit with reformatescontaining a high carbon monoxide ratio that is incompatible with thecells of subassembly 13 but compatible with the cells of subassembly 12.The reformate produced in this way, rich in carbon monoxide, is sent tothe cells of subassembly 12, valve 17 being closed. It will also benoted that valve 19 is closed and that switch 20 is open. Thus onlysubassembly 12 is supplied with hydrogen and with air, and it generateselectric energy. The thermal losses due to electricity generation bringsubassembly 12 from ambient temperature to its optimaloperating-temperature range.

Cooling circuit 21 is inactive and pump 26 is stopped as long as atemperature below a first threshold, such as 80° C., has not beenreached by subassembly 12, the temperature of which is measured bysensor 24. As soon as temperature sensor 24 measures a temperature abovethe first threshold, central unit 22 commands pump 26 to beginoperating, thus circulating a cooling fluid in subassemblies 12 and 13.Heat transfer takes place from subassembly 12 to subassembly 13, whichprogressively heats up.

When temperature sensor 25 of subassembly 13 detects a temperature abovea second threshold, which may be a threshold higher than the firstthreshold used for subassembly 12, such as 120° C., and when temperaturesensor 23 of reformer 10 detects a temperature above a third threshold,such as 800° C., central unit 22 commands pilot-controlled valves 17 and19 to open. The temperature condition on reformer 10 guarantees areformate in which the carbon monoxide concentration is compatible withsubassembly 13 and the temperature condition on subassembly 13guarantees good energy efficiency thereof. Switch 20 is closed.Subassembly 13 is supplied with fuel and oxygen carrier and is connectedto the electric power system.

If the energy consumption of the power train (not illustrated) isgreater than the electric power generated by subassembly 13 during thetemperature-rise phase, supplementary power can be supplied by a battery(not illustrated).

The fact that the fuel-cell stack is composed of two subassemblieshaving different characteristics and made of different materials makesit possible to lower the overall cost of the stack, because therequirements due to starting are applied to only one of thesubassemblies, while the other subassembly can be of less robustconstruction and be provided with a smaller quantity of catalysts.

Of course, the pile can also be subdivided into a larger number ofsubassemblies, such as three or four, which could be brought intooperation successively as a function of the composition of the reformatedelivered by the reformer.

The invention also permits great modularity in the arrangement of thecells of the fuel-cell stack, and above all it permits fast starting ofthe vehicle equipped with such a fuel-cell stack, thus greatlyincreasing the pleasure of using of such a vehicle and potentiallyfacilitating the marketing thereof.

Finally, the invention permits an improvement of the energy efficiencyof the stack, because the second subassembly generates electric energyonly when it has reached its optimal temperature range, in which itexhibits high efficiency.

1- A method for starting a fuel-cell stack, comprising a plurality ofcells supplied by a reformer (10), in which method, when the reformer iscold, a first subassembly (12) of cells of the fuel-cell stack issupplied with reformates and then, when the reformer is hot, the firstand second subassemblies of cells of the fuel-cell stack are supplied,the cells of the first subassembly being optimized for operation with acold reformer and the cells of the second subassembly (13) beingoptimized for operation with a hot reformer. 2- A method according toclaim 1, in which the cells of the second subassembly are supplied whenthe said cells are at an appropriate operating temperature. 3- A methodaccording to claim 1 or 2, in which a cooling circuit (21) common to thefirst and second subassemblies of cells of the fuel-cell stack isactivated when the temperature of the first subassembly of cells reachesa temperature threshold. 4- A fuel-cell stack system, comprising afuel-cell stack (8) provided with a plurality of cells and a reformer(10) capable of supplying hydrogen from a hydrocarbon fuel,characterized in that it comprises a first subassembly (12) of cellsoptimized to operate with a cold reformer and a second subassembly (13)of cells optimized to operate with a hot reformer, and means forsupplying the second subassembly of cells as a function of the reformertemperature. 5- A system according to claim 4, characterized in that itcomprises means of supplying the second subassembly of cells as afunction of the temperature of the said second subassembly. 6- A systemaccording to claim 4 or 5, characterized in that it comprises a coolingcircuit (21) common to the first and second subassemblies of cells, insuch a way that the heat released by the first subassembly (12) of cellsheats the second subassembly (13) of cells when the latter is shut down.7- A system according to any one of claims 4 to 6, characterized in thatit comprises a pilot-controlled valve (17) mounted on a reformate-supplyconduit of the second subassembly of cells, a pilot-controlled valve(19) mounted on an air-supply conduit of the second subassembly ofcells, and an electronic switch (20) mounted on an output conductor ofthe second subassembly of cells. 8- A system according to any one ofclaims 4 to 7, characterized in that it comprises a central unit (22)provided with means to run a software routine, with a memory and with atleast one software routine stored in the memory, the software routinecomprising a module to activate a cooling circuit (21) when thetemperature of the first subassembly of cells reaches a temperaturethreshold. 9- A vehicle comprising a power train with electric motor anda fuel-cell stack system according to any one of claims 4 to 8.